Correct tire pressure is an often-overlooked part of maintaining optimal performance and assuring safe operation of a car. Correct tire pressure affects the car's gas mileage, its ability to grip the road, and the longevity of the tires themselves.
Ideally, tire pressure should be checked frequently. However, many people do not do so. And many of those who do check simply look at the tire and attempt to judge, from its appearance, whether the tire pressure is correct. Others may press a finger against the tire, as if they were checking a fruit for ripeness, and based on that feel of the tire, proclaim the tire is properly inflated.
These methods are generally not accurate. Proper checking of tire pressure requires measuring the tire pressure and comparing it against a desired tire pressure. This raises another difficulty. Different makes and models of cars have different recommended tire pressures. These are usually printed on a sticker on the driver's door jamb. However, not everybody knows this. Moreover, in some cases, the units of pressure that are printed in the door jamb are inconsistent with the units on a tire-pressure gauge, thus requiring a unit conversion.
To make matters more difficult, the optimal tire pressure can depend on the environment and on road conditions. The information printed on the driver's door jamb represents an optimal pressure for one set of conditions only.